The present invention relates to a method and apparatus for feeding a coating mix, size or other web treating agent to a paper or board web processing apparatus in which the agent is applied to the web surface. More specifically, the invention relates to the machine circulation of a sizing press or coating station.
In the coating of a paper or board web, to the surface of a base sheet are applied different kinds of treating agents serving to improve the strength and printability of the web. Most conventionally, a surface sizing agent and various coating mixtures are employed. The web treating agent can be applied in various manners to the web being processed. In the art of coating, the coater equipment may be selected, e.g., from the groups of roll coaters such as film-transfer coaters, short-dwell coaters and spray or jet coaters. Using the applicator apparatus, an excess amount of coat is applied to the base sheet surface and the applied coat is smoothed to a desired thickness by a doctoring device. In surface sizing, the treating agent used for treating the web penetrates into the base sheet, thus disposing with any need for doctoring. In modern coaters, the applicator apparatus is designed to allow flooding of the coating mix over the confining boundary element located at the coater incoming side such as a blade or a rod. The purpose of the continuous flooding is to prevent the entry of air travelling along with the web into the applicator chamber and to provide a linear web wetting line at the incoming side, whereby a constant coating mix dwell time profile in the cross-machine direction over the web is attained and the surface profile of the coated product becomes smoother. To achieve a stable overflow, an excess amount of the coating mix is pumped into the applicator chamber and the excess coating mix flooding at the incoming side over the applicator chamber wall or the confining boundary element is collected and circulated back into the machine tank. The return flow of coating mix is passed via a strainer, is purged free from entrained air and then is returned to the coating mix circulation.
The pumping of excess coating mix and the open structure of the return circulation involve a number of problems. The greatest complications are caused by the air and foreign matter entrained in the circulating treating agent. Air can become entrained in the circulating treating agent in the return pipe which must be overdimensioned so that it will never become fully plugged by the treating agent. The coating mix circulated via the applicator chamber meets the web surface during application, whereby it gathers fibers detached from the web surface. In concurrent machine circulation systems of coating mix, removal of air and foreign matter from the treating agent is accomplished by air traps and strainers that must be dimensioned for high volumetric flows, because the proportion of the return flow may be up to 90% of the overall flow of the treating agent directed to meet the web surface. Due to the large volume of the coating mix flow, also the other parts of the coater equipment must be made large thus requiring a large footprint. As coating mixes especially have a high viscosity and are difficult to handle, their straining and entrained air purging are clumsy to arrange and require costly equipment of high efficiency. The equipment must have a high capacity that makes the implementation of the machine circulation of coating mix an expensive investment when using conventional arrangements. Removal of air and foreign matter from the circulating coating mix, however, is mandatory inasmuch both of these disturbing factors will otherwise cause defects on the coated web of paper or board. The large volumetric flow also means a large volume of coating mix contained in the machine circulation at a time. Furthermore, the circulation system of the web treating agent may entail spaces that are supportive to the growth of bacterial cultures. This is a significant problem when using organic size formulations, because the size containing warm water and air will be readily contaminated by microbes. To avoid the growth of microbial cultures, biocides must be added to the sizing used in the size circulation thus causing extra costs and hampering the processing of waste waters containing such web treating agents.
It is an object of the present invention to provide a method and apparatus suited for feeding a web treating agent to such web processing apparatuses employed in the manufacture of paper and board that do not have a return circulation arrangement for the treating agent flooding from the applicator.
The goal of the invention is achieved by way of adapting a shunt flow facility to the coating mix applicator chamber and then measuring the pressure head of the applicator chamber and adjusting the inlet flow rate of the coating furnish so as to keep the applicator chamber head at a desired level.
According to a preferred embodiment of the invention, the positive inlet flow pressure, that is, the infeed pressure to the applicator chamber, is adjusted to a proper level by utilizing the hydrostatic head of the treating agent flowing from a machine tank disposed at a suitable height in regard to the applicator chamber.
The invention offers significant benefits.
The most important benefit of the invention is that the web treating furnish can be kept entirely fresh and free from entrained air, whereby there is no need for straining and purging of entrained air and yet the quality of the treated/coated web surface remains defect-free. The greatest benefit of the system having no open return circulation is attained in size presses which have no return circulation for the excess size doctored away from the surface of the treated web or, alternatively, in coating methods which directly apply to the paper or board sheet surface the correct amount of coat needed for a desired final coat thickness, thus disposing with the doctoring step. Such coating techniques are feasible at least by means of some film-transfer coaters and jet applicators obviously, any system involving a doctoring step must subject the return circulation of the treating agent to straining and/or purging of entrained air prior to the reuse of the treating agent removed by doctoring. Another significant benefit is the substantial reduction of the footprint required by the equipment and the system and the lesser amount of treating agent required in the machine circulation. As no purging of entrained air or straining of the treating agent is needed, the arrangement is more cost-effective to implement. The smaller amount of circulating treating agent contributes to easier temperature control. The number of valves required in the machine circulation is reduced over that needed in a conventional system and the transducer installation and control instrumentation of the machine circulation is easier to implement. Inasmuch as the piping of the treating agent circulation system is always running full of treating agent and free from air, there is no risk of the drying of the agent in the piping and accumulating on the walls thereof, wherefrom it could then flake off as clumps that become entrained in the material flow thus possibly entering the applicator chamber and landing on the web with the applied coat. Elimination of this hazard is a significant advantage in coaters. Furthermore, the growth of microbial cultures in the treating agent is prevented by the anaerobic conditions, thus disposing with the need for biocide use. Owing to the minimal footprint requirement, the equipment of the machine circulation system can be mounted on the machine level, while conventional arrangements have the machine tank placed on the basement level and the strainers on the intermediate or machine level, thus necessitating a much longer overall length of machine circulation piping than is now possible in an arrangement according to the present invention.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawing. It is to be understood, however, that the drawing is intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.